Calixarene-Mediated Synthesis of Cobalt Nanoparticles: An Accretion Model for Separate Control over Nucleation and Growth

Abstract

The nucleation and growth of crystalline cobalt nanoparticles (Co NPs) under solvothermal conditions can be separated into distinct stages by using (i) polynuclear clusters with multivalent capping ligands to initiate nucleation, and (ii) thermolabile organometallic complexes with low autonucleation potential to promote crystalline growth. Both nucleation and growth take place within an amorphous accretion, formed in the presence of polyvalent surfactants. At the pre-nucleation stage, a calixarene complex with multiple Co2–alkyne ligands (Co16–calixarene 1) undergoes thermal decomposition above 130 °C to form “capped cluster” intermediates that coalesce into well-defined Co nanoclusters, but are resistant to further aggregation. At the post-nucleation stage, a monomer (pentyne–Co4(CO)10, or PTC) with a low thermal activation threshold but a high barrier to autonucleation is introduced, yielding ε-Co NPs with a linear relationship between particle volume and the Co mole ratio ([Cofinal]/[Coseed]). Co nanocrystals can be produced up to 40 nm with a 10–12% size dispersity within the accretion, but their growth rate depends on the activity of the supporting surfactant, with an octapropargyl calixarene derivative (OP-C11R) providing the most efficient transport of reactive Co species through the amorphous matrix. Post-growth digestion with oleic acid releases the Co NPs from the residual accretion, which can then self-assemble by magnetic dipolar interactions into flux-closure rings when stabilized by calixarene-based surfactants. These studies demonstrate that organometallic complexes can be designed to establish rational control over the nucleation and growth of crystalline NPs within an intermediate accretion phase.

Selected applications of calixarene derivatives

“…It is noteworthy that the applications of calixarenes are diversified; one may mention here the preparation of calixarene-based ionic liquids, 101,102 and formation of metal NPs, 103,104 today intensively studied. Although due to the large amount of works dealing with calixarenes, [105][106][107][108][109][110] in this review only selected examples are presented, one might hope that the recent reports here cited will help towards a better understanding of this interesting field of chemistry and related sciences.…”

An aggregative growth process for controlling size, shape and composition of metal, alloy and core–shell nanoparticles toward desired bioapplications

“…aggregative growth in the control of size has also been recognized in studies of other metal nanoparticles. Examples include calixarene-mediated nucleation and growth of cobalt nanoparticles under solvothermal conditions, 45 seeded growth of Au NPs in triblock copolymers demonstrating bimodal size distributions, 46 and Au nanocrystal growth under thermal treatment as a result of the difference in melting propensity between small and large sizes. 47 The latter shows that the smaller seeds (<3 nm) are melts with formation of liquid-like particles that prefer to adhere to the {111} faces of the larger particles.…”

Synthesis of cobalt, palladium, and rhenium nanoparticles

“…Further increasing the Ir:Co ratio to 0.025 and 0.050 had a negligible effect on the nanorod diameter but reduced their length from 100-300 nm to 50-200 nm and 40 nm, respectively. Chen et al [59] illustrated that nucleation and growth of crystalline NPs can be governed by organometallic complexes through an intermediate agglomeration phase. This two-step solvothermal technique included a prenucleation step and a growth step.…”